The mitochondrial oxidation of long chain fatty acids is regulated in large part at the level of carnitine palmitoyltransferase I (CPT I) located on the mitochondrial outer membrane. This enzyme exists in at least two isoforms, the liver (L) and muscle (M) types, which are expressed to different extents in any given tissue. We have recently discovered that the M-CPT I gene is robustly expressed in the testis of the rat, mouse and human. More specifically, we have shown that the production of M-CPT I mRNA and protein is restricted to the developing germ cell at the early meiotic stage of development, whereas non-germ cells of the testis express only L-CPT I. By contrast, in the ovary, L-CPT I mRNA is abundant while the M-CPT I gene remains dormant. Because the role of fatty acid oxidation in general, and of CPT I in particular, in germ cell development and function have received little if any attention to date we would like to try to break through this impasse. Accordingly, we have two specific aims. First, to create a mouse line in which the M-CPT I gene has been disrupted selectively in developing sperm (using the Cre/loxP technology) and to determine the effect of the knockout on the animal's ability to transmit the mutant gene to progeny. The possible impact of this transmission on heart and skeletal muscle function will also be examined. Second, to determine which cell types in the testis and ovary express L-CPT I using the techniques of in situ hybridization, immunocytochemistry and biochemical analysis. If, as we suspect, M-CPT I turns out to be crucial for normal sperm development, we will determine if spermatocytes lacking the enzyme progress to a non-functional spermatozoan stage or become arrested earlier in development. If the latter, a detailed phenotypic analysis of the abnormal germ cells will be performed. Such information would fill a major gap in our understanding of sperm physiology and could become relevant to the issue of male infertility, most cases of which have defied explanation to date. It would also throw light on whether we are ever likely to find cases of inherited human M-CPT I deficiency (none have been reported to date) and, if so, whether the faulty gene can be passed through the male germ line. Finally, should CPT I prove to be important in germ cell function, important questions would be raised concerning the proposed use of CPT I inhibitors in the treatment of diabetes and postischemic heart injury. On the other hand, a reversible CPT I inhibitor that could be targeted to the testis or ovary might well find use as a male or female contraceptive agent.